The Muscular System
Interactions of Skeletal
Muscles
10
 Muscle tissue includes all
contractile tissue  skeletal
muscles take center stage!
Interactions of Skeletal Muscles in the Body
 The arrangement of muscles permits them to work
together or in opposition to achieve a variety of
movements.
 Muscles can only pull then can never push!!!
Interactions of Skeletal Muscles in the Body
 Origin – muscle attached to
the immovable (or less)
movable bone.
 Insertion – muscle attached to
the movable bone.
 Ex. Spring on a screen door.
Interactions of Skeletal Muscles in the Body
 As a muscle shortens:
 The insertion moves
towards its origin.
 Whatever one muscle or
group of muscles does 
another muscle or group of
muscles must “undo” the
action
 Flex the elbow  extend
the elbow
Interactions of Skeletal Muscles in the Body
 Skeletal muscles work together or in opposition
 A muscle cannot reverse the movement it produces
 Another muscle must undo the action
 Muscles with opposite actions lie on opposite sides
of a joint
Interactions of Skeletal Muscles in the Body
 Prime mover
 Agonist
 Provides major force for a
certain movement
Interactions of Skeletal Muscles in the Body
 Antagonist
 Muscles that oppose, or
reverse, a particular
movement.
 Can also help to regulate
the action of the prime
mover by contracting to
provide some resistance
 When prime mover is
active  relaxed and
stretched
Interactions of Skeletal Muscles in the Body
 Prime movers and
antagonists lie on opposite
sides of the bone!
 Depending on the
movement:
 A prime mover can become
an antagonist
 An antagonist can become
a prime mover
Interactions of Skeletal Muscles in the Body
 Synergist
 Syn = together; erg = work
 Help prime mover:
 Add extra force to same
movement
 Reduce undesirable or
unnecessary movements
that might occurs at prime
mover contracts.
Interactions of Skeletal Muscles in the Body
 Synergist
 Example – if a muscle crosses two or more joints
 movement occurs at all the joints  stabilize so
movement doesn’t occur.
 Example – Muscles that cause finger flexion 
found in forearm
Interactions of Skeletal Muscles in the Body
 Fixator
 A synergist that immobilizes a
bone or a muscle’s origin
 Example – scapula
 Held to the axial skeleton only by muscles, but is also freely
movable.
 Fixators immobilize the scapula so that only the desired movements
occur at the shoulder joint.
Interactions of Skeletal Muscles in the Body
 Prime movers, antagonists,
and synergists work
together to produce smooth
coordinated, and precise
movements
 A muscle can switch roles
depending on the
movement!